Containers of crystallizable polymer construction—e.g., PET construction—are conventionally fabricated in an injection blow molding operation, in which a preform is fabricated by injection molding, and the preform is then subjected to a blow molding operation to form the container. The container finish is formed to final geometry or substantially final geometry in the injection molding operation, and generally is of amorphous polymer construction. The body of the container is subjected to stretching during the blow molding operation so that the body is of oriented polymer construction. It has been proposed to strengthen and rigidify the finish portion of the container by crystallizing all or selected portions of the neck finish. See, for example, U.S. Pat. Nos. 4,618,515, 4,928,835 and 6,217,818. The uncrystallized portions of the container body and finish are typically translucent, while the crystallized portion(s) of the finish contain spherulite that has a milky opaque appearance.
The term “crystallizable polymer” in the present application refers to any polymer that is capable of being crystallized. These polymers include not only polyesters such as PET, which is the crystallizable polymer most typically used to make containers, but also polyolefins such as polyethylenes and polypropylenes, polycarbonates and polyamides such as nylons. Thus, although specific embodiments of the invention are disclosed in conjunction with polyesters—i.e., PET—the invention is by no means limited specifically to this polymer or family of polymers.
Likewise, when referring in the present application to preforms or containers that are at least partly of crystallizable polymer construction, or to preforms or containers of crystallizable polymer construction, such language refers not only to containers that are entirely of crystallizable polymer construction, but also to multilayer containers in which inside and outside layers are of crystallizable polymer construction. Other intermediate layers of the container or preform may include barrier materials against migration of gases through the container walls, layers of process regrind and/or layers that include post consumer resin. These intermediate layers may be included only in the body of the container, such that the container finish is of unlayered polymer construction, or may extend into the finish of the container. See, for example, U.S. Pat. Nos. 4,550,043, 4,609,516, 4,781,954, 4,990,301 and 5,098,274.
It has been proposed to crystallize the entire finish of a PET container to improve dimensional stability of the container finish. However, such complete crystallization may induce shrinkage of the finish as a result of crystallization, which is unacceptable in the threaded section of the finish to which a container closure will ultimately be secured. Conversely, a completely amorphous finish is subject to softening and movement as the container is hot-filled and capped because the temperature of the container may rise above the glass transition temperature of the plastic. It is therefore an object of the present invention to provide a technique for selectively crystallizing portions of the container finish that substantially reduces or eliminates shrinkage of the finish as a result of crystallization, provides desired stability in the finish sealing and thread area where the container closure will ultimately be secured, and/or that reduces consumption of energy as compared with crystallization of the entire finish area. Other and more specific objects of the invention are to provide a preform for blow molding a hollow plastic container, a hollow plastic container, and a method for forming a hollow plastic container in which the container finish is selectively crystallized as described above.
These and other objects are achieved in accordance with the present invention by selectively crystallizing the neck finish of the preform or container in such a way as to achieve substantial crystallization in at least a first portion of the finish, substantially no crystallization in a second portion of the finish, and graded crystallization between the first and second portions of the finish. The term “graded crystallization” refers to a gradual transition between substantial crystallization and substantially no crystallization, as distinguished from a sharp or distinct non-graded pattern of contrast between crystallized and uncrystallized portions of the neck finish. The gradation of crystallization is continuous, and may be either linear or non-linear with distance.
The present invention embodies a number of aspects, that may be implemented separately from or in combination with each other.
In accordance with a first aspect of the invention, a preform for blow molding a hollow plastic container, which is at least partly of crystallizable polymer construction, includes a finish and a body integral with the finish. The finish has an end portion remote from the body, a capping flange adjacent to the body, and a mid portion between the end portion and the capping flange. The end and mid portions form a continuous outer surface and a continuous inner surface, with one of the inner and outer surfaces being crystallized throughout its length and the other of the inner and outer surfaces including at least a portion that is essentially uncrystallized. Crystallization in at least the mid portion of the finish is graded between the crystallized surface and the uncrystallized portion of the other surface. In accordance with another aspect of the present invention, the end and mid portions form a continuous outer surface that is crystallized throughout its length, and a continuous inner surface at least a portion of which is essentially uncrystallized. Crystallization in at least a mid portion of the finish is graded between the outer and inner surfaces, and between the mid portion of the finish and the capping flange. In accordance with a third aspect of the invention, at least the end portion of the finish is crystallized, and crystallization in the finish is graded from full crystallization at the end portion.
In accordance with yet another aspect of the present invention, a hollow plastic container of crystallizable polymer construction includes a finish and an integral body. The finish has an end portion remote from the body, a capping flange adjacent to the body, and a mid portion extending between the end portion and the capping flange. The end and mid portions form continuous outer and inner surfaces, with one of the outer and inner surfaces being crystallized throughout its length and the other of the inner and outer surfaces including at least a portion that is essentially uncrystallized. Crystallization in at least the mid portion of the finish is graded between the crystallized surface and the uncrystallized portion of the other surface. In accordance with another aspect of the invention, the end and mid portions of the container finish form a continuous outer surface that is crystallized throughout its length, and a continuous inner surface at least a portion of which is essentially uncrystallized. Crystallization in at least the mid portion of the container finish is graded between the outer and inner surfaces and between the mid portion and the capping flange. In accordance with a further object of the invention, at least the end portion of the finish is crystallized, and crystallization in the finish is graded from full crystallization at the end portion of the finish.
Yet another aspect of the present invention provides a method of making a hollow plastic container, which includes molding a preform of crystallizable polymer construction and blow molding the preform to form the hollow plastic container. Either prior to or subsequent to blow molding the preform, the finish is crystallized at one of the inner and outer surfaces of the finish in such a way that one surface is crystallized throughout its length and the other surface includes at least a portion that is essentially uncrystallized, with crystallization in at least the mid portion of the finish being graded between the surfaces. In accordance with a further aspect of the invention, crystallization of the finish of either the preform or the container is such that the outer surface of the finish is crystallized throughout its length and at least a portion of the inner surface is essentially uncrystallized. Crystallization in at least the mid portion of the finish is graded between the outer and inner surfaces and between the mid portion and the capping flange. In accordance with yet another aspect of the invention, crystallization at the finish is such that at least the end portion of the finish is crystallized, and crystallization in the finish is graded from crystallization at the end portion.